1. Field of the Invention
The present invention relates to a gas laser device, and more particularly to a gas laser device in which laser light is generated by applying a high-frequency voltage across electrodes to achieve electrical discharge.
2. Description of the Related Art
FIG. 1 shows the longitudinal cross-sectional structure of a so-called cross-flow type of gas laser device in which the gas flow is orthogonal to the output optical axis. In FIG. 1, a metallic inner flume 2 made of, for example, stainless steel, iron or aluminum with a U-shaped cross-section is provided inside an outer flume 1 made of, for example, stainless steel, iron or aluminum with a rectangular cross section. A first tabular dielectric 3a made of ceramic is hermetically attached to the central portion in the top surface of the outer flume 1. A second tabular dielectric 4a is hermetically attached to the inner flume 2 in such a way as to close the opening in the top surface. And the arrangement is such that a discharge gap 5 with a predetermined spacing exists between these dielectrics 3a and 4a. Further, a first electrode 3b is attached to the central portion in the top surface of the first dielectric 3a to form a first dielectric electrode 3. And a second electrode 4b is attached in the central portion in the bottom surface of the second dielectric 4a to form a second dielectric electrode 4 which constitutes a pair with the first dielectric electrode 3. Moreover, the first electrode 3b and second electrode 4b are connected to an a.c. power source 6.
The laser gas is enclosed at the desired pressure in the gap between the outer flume 1 and the inner flume 2, and the inside of the inner flume 2 is connected to the outer atmosphere. A blower 7 for circulating the laser gas in the direction of the arrow A and a heat exchanger 8 for cooling the laser gas after it has flowed through the discharge gap 5 are placed at the bottom on the inside of the outer flume 1.
When an a.c. voltage from the a.c. power source 6 is applied across the first electrode 3b and the second electrode 4b in the laser device with this configuration, an a.c. discharge occurs in the discharge gap 5 via the first dielectric 3a and the second dielectric 4a, the laser gas flowing in the discharge gap 5 is excited and the laser light 9 is produced in a direction orthogonal to the surface of the paper.
As shown in model form in FIG. 2, the physical composition for the discharge in the discharge gap 5 comprises a positive column 10 and boundary layers 11 in the vicinity of the first and second dielectric electrodes 3 and 4. The behavior of the boundary layer 11 depends on the discharge frequency.
Next, in order to raise the laser oscillation efficiency (conversion efficiency from discharge input to laser output) it is necessary to raise the frequency of the a.c. power source. In this case, the a.c. power source of the vacuum tube type is to be used. The efficiency of the a.c. power source (conversion efficiency from 200 V a.c. input power to high frequency output power) is up to 70% in the case of C class amplification type, and is usually 55%. As a result, it is impossible to raise the overall efficiency of the gas laser device (conversion efficiency from 200 V a.c. input power to the laser output power). Besides, the size of the a.c. power source is large, because the power source of the vacuum tube type is used.
Contrary, in order to raise the power source efficiency (maximum about 90%) and to reduce the size of the a.c. power source, it is necessary to use the solid state device in the a.c. power source and to make the frequency of the a.c. power source low (100 kHz). In this case, the laser oscillation efficiency will be reduced, as a result the overall efficiency of the gas laser device will not be raised.
Namely, the power injected to the positive column 10 contributes to laser excitation but the power injected to the boundary layer 11 does not contribute to laser excitation. Gas laser devices with a conventional configuration, however, do not take into account power losses in the region of the boundary layers 11 and therefore experience problems in that the laser oscillation efficiency is low since power loses in the region of the boundary layer 11 account for a large proportion of the overall discharge input.
Moreover, as the output frequency is low which will not give rise to the phenomenon of electron trapping and will make the ignition voltage high. As a result, laser pulse characteristic is also not good. The conventional gas laser device described above suffers from the problems as described above.
Next, the operation of the a.c. power source 6 will be described. The a.c. power source 6 here carries out a continuous (CW) operation and pulse operation. In order to improve the discharge ignition characteristic for the pulse operation, the discharge is ignited by a preliminary ionization means such as Simmer discharge even when the laser pulse is off. The power-source output from the a.c. power source 6 as shown in FIG. 3 therefore has alternate Simmer discharge periods T1 and main discharge periods T2.
The load on the a.c. power source 6 is thus lessened when carrying out the laser pulse operation by reducing the difference between the voltage in the Simmer discharge period T1 and the voltage in the discharge period T2 generated by the a.c. power source 6.
However, conventional gas laser device with the configuration described above suffers from problems as described below.
Because the device has Simmer discharge, the discharge always has to be ignited even when the laser pulse operation is not being carried out and the efficiency is reduced.